This invention has for a specific objective the production of gradients in the refractive index of glass articles through the use of ion exchange techniques, such technology being applicable to the optical engineering field, particularly the making of high performance fiber optic components such as gradient index (GRIN) lenses.
The current interest in making high performance fiber optic components has led to a resurgence of efforts to further pursue optical engineering technologies. One such technology being developed combines ion exchange techniques with photolithography for integrating optical wave-guides in a glass substrate. Optical wave-guides can be embedded in a glass substrate to create a wide variety of optical circuits and passive optical functions for devices such as splitters, stars, wavelength division multiplexers, and optical power taps. These functions are readily integrated into a single glass substrate to facilitate component miniaturization and controlled performance. Such research was presented by Kaps, Karthe, Muller, Possner, and Schreiler in “Glasses for Optoelectronics,” ECO Proceedings, Paris, France, Vol. 1128, Apr. 24-27, 1989.
Ion exchange, a technique for producing gradients in the refractive index of glass articles, has been in use since the early sixties. The essence of this method lies in the exchange of ions having different polarizabilities, viz., exchanging one alkali ion for another. For example, U.S. Pat. Nos. 3,524,737 and 3,615,322 describe techniques for strengthening glass whereby the sodium ion in glass is replaced by potassium and copper ions, respectively. Similarly, U.S. Pat. No. 3,615,323 describes a similar glass strengthening technique, with the sodium ion being replaced by a lithium ion. Modest changes in refractive index are achieved by such exchanges.
Presently, thallium has commonly been chosen over other elements as a doping ion to create regions with a higher refractive index. Large changes in the refractive index of glasses have been achieved by the ion exchange of thallium; however, the use of thallium is limited to some extent by its toxicity. Nevertheless, thallium is the ion most often used today in ion exchange processes in spite of its inherent toxicity problems. The silver ion is very polarizable and is expected to produce a change in refractive index comparable to that produced by tantallum and it is free of toxicity problems.
The potential benefit of silver in the production of high index silicate glasses has not been fully realized because it has been observed that the introduction of more than minimal amounts of silver into a silicate glass by ion-exchange techniques invariably led to extensive chemical reduction of the silver and often led to the formation of macroscopic particles of metallic silver. The direct-consequence when this occurs is an unacceptable decrease in the concentration of ionic silver in the glass. In other cases, the reduction caused the formation of large colloids of silver with an attendant unacceptable increase in attenuation in the optical path of the lens. The intense color which characterizes the for Nation of large colloids when silver is reduced is unacceptable for certain applications where an essentially colorless, transparent glass is required.
In co-assigned, U.S. Pat. No. 5,007,948, we demonstrated that silver is not reduced when it is added by ion exchange to a glass which does not contain non-bridging oxygen atoms and does not contain polyvalent ions. The introduction of silver to a glass rod by ion exchange produces a body having a higher refractive index on the surface than that in the interior. However, for many optical applications, it is desirable to have the higher index on the interior. One method which has been demonstrated for achieving this involves two separate ion-exchange steps. In the first, a high uniform concentration of silver is introduced throughout the body of the optical device (such as glass). In the second ion exchange step, a less polarizable ion, such as sodium, is introduced to the surface layer of the device. In order to obtain a uniform concentration of silver, the time required to complete the first ion-exchange process is very long.
The fabrication of an optical device with a low refractive index on the surface would be more convenient if it were possible to melt a glass containing a high concentration of silver so that only a single relatively short ion exchange step will be required to introduce less polarizable ions to the surface of the device. Thus, there continues to be a need for different methods for making high performance fiber optic components such as gradient index lenses.